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Astron. Astrophys. 319, 967-972 (1997)

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Effect of chromospheric activity on the mean colours of late-type stars

P.J. Amado and P.B. Byrne

Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland

Received 2 July 1996 / Accepted 2 August 1996


In this paper we show, for the first time, the effect of activity on the (U - [FORMULA] vs. (B - [FORMULA] colour-colour diagram for a large sample of late type stars. We clearly see that for single-lined spectroscopic RS CVn systems there exists an ultraviolet excess over that recorded for quiescent stars of the same spectral type. The same effect is seen in main-sequence stars but to a lesser extent. From simple arguments we demonstrate that the cause is unlikely to be unresolved flaring, the combined effect of strong chromospheric emission lines, or X-ray/UV back-heating of the photosphere. White-light faculae are a reasonable candidate for the effect however. In our investigation we have not taken into account the effect that metallicity might have on the colour indices.

Key words: stars: activity – stars: fundamental parameters – stars: late-type

Send offprint requests to: P.J. Amado

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998